How to never be disappointed again

We spend lot of time in our lifes being disappointed by the actions of others. We are being repeatedly hurt by their "selfish" actions without realizing why are we sad in the first place and maybe that it is us who is a) selfish and b) poor communicator. We all create models of expectations around what others should or shouldn’t do, based on our own "selfish" beliefs, our own little world. Then we go on in our life’s and we are surprised that the expectations we have set for us and others are not being met and that leaves us sad and disappointed. One principle I have created for me is: "never expect for something to automatically happen". Also recently, I have added new principle that says:

"you can only expect there, where deal was made"

This is super strong for me and takes me further from just people but also future in general. Lets say you wish that you will lose that last couple of kilos on your belly that are so hard to lose.  That is that last 5% of imperfection that costs you 95% of effort. Now, what if you cant lose that? What if no matter how hard you try, you have simply kind of a physique that wont let you do that. Will you be sad? Have you made a deal with your belly or "god of fitness" that says," I promise you that if you do x y and z, you will get rid of that last 5% on your belly. I don’t think so. So stop beating yourself up trying to be perfect, you are not. And do not expect life to be perfect, because we are mammals, imperfect creatures.

So my main takeaway is this. Next time you will be disappointed, ask yourself two things. 1) did I make a deal that both parties agreed on? 2) did I communicate that deal clearly to other person? I the answer is no, then relax and try to do it next time. And that is a first key towards the way how no to be disappointed gradually ever again.